The use of heat pumps augmented by solar energy or air convectors for the purpose of heating building structures has been known in the past. However, usage of such energy has been on a limited basis and in only certain geographical areas where either the average temperature is usually quite high as, for example, in Florida wherein air-to-air heat pumps and water-to-air heat pumps have been practical and economical or in areas where the sun energy is plentiful as in the United States south and southwest areas wherein water heating by solar energy has been used for many years.
However, in temperate and colder climates it has not been practical to use such techniques. Heat pumps perform very efficiently at outside ambient temperatures of 45, 35 or even 25 degrees Fahrenheit. However, the efficiency of heat pumps drops off as temperatures become lower, and approximately below 20.degree. F. evaporator temperature, a heat pump is not very efficient. In some climates where an air-to-air heat pump is not practical, water-to-air heat pumps have proven satisfactory. However, such apparatus requires an adjacent river, creek, or well to be available for evaporator coils. The availability of such a water source is, of course, quite limited, thereby severely restricting the extent of use of water-to-air systems.
Also, earth-to-air heat pump systems have been used. In such cases, an evaporator coil is buried, for example, three to six feet or more deeply in the ground, to pick up heat from the ground, which is normally at a higher temperature than the air temperature, and pass this heat through a heat pump system to the air to be heated. Such a system may be quite costly because a very large evaporator coil construction is needed to contact enough ground area so that the evaporator temperature will not fall below 25.degree. to 30.degree. F., which is the temperature below which the heat pump will not operate efficiently.
In my copending United States patent application, Ser. No. 532,156, filed on Dec. 12, 1974, I have disclosed a structure for collecting solar energy through a solar collector during periods when such energy is available and storing it directly in the earth surrounding an evaporator which is also buried in the earth. The evaporator forms part of a heat pump system. With such a structure, when there is a day or several days when the sun shines, an excess of solar heat will be transmitted to the earth surrounding the heat pump evaporator. Cloud diffused solar energy in appreciable amounts can be stored even on cloudy days. The earth itself acts as a storage agent. The earth serves as a natural and inexpensive storage area. Many tons of earth are available at low cost to store a considerable amount of heat. Once such a mass of earth is heated, if requires many days for it to cool to its normal temperature. A similar system is disclosed in U.S. Pat. No. 2,529,154. There have been other attempts in the past to provide solar or heat pump systems for heating as are illustrated in U.S. Pat. Nos. 2,559,870; 3,262,493; 1,101,001; 1,130,870; 1,683,434; and 3,178,113, and also an article appeared in the WASHINGTON POST on Oct. 12, 1974, entitled "Heat Pump a Key to Solar Heating".
In my present invention, I have provided a novel way for utilizing at least two banks of heat collectors. Each bank is an independent heat exchange relationship with an evaporator which is buried in the earth. Additionally, novel means are provided for mounting of coils buried in the earth. A further novel feature of the invention is the provision of a water tank connected to the outlet of the evaporator which in addition to functioning as a heat reservoir, acts as a buffer to prevent excessive liquid refrigerant from returning to the compressor of the heat pump system when unusual conditions of operations are encountered.